| A series of Al-0.34%Mg-1.05%Si-0.08%Cu sheet samples were solutionized at550℃for30minutes, then were pre-strained by0%,2%,5%and pre-aged at170℃for0,3,5and10minutes, finally baked at180℃for30min after240hours natural ageing.The effects of pre-treatment on natural ageing, bake hardening response and plasticitywere investigated using vickers hardness test, tensile test and conductivity test.Differential Scanning Calorimetry (DSC), X-Ray Diffraction (XRD) and opticalmicrostructure etc. was conducted to reveal corresponding precipitation mechanismsduring natural aging and bake hardening response (BHR). The influence of natural agingon microstructure and mechanical properties were analyzed. The effects of pre-aging,pre-straining, pre-aging and pre-straining on suppressing natural aging and enhancingbake hardening response was compared to obtain the best technological parameter. Thesensitivity of pre-straining and pre-ageing on mechanical properties was evaluated bytwo-factor analysis of variance and the activation energy of′′phase was valued.①The strength before bake hardening was92.48MPa which was too high due tothe Clustering during natural aging, the conductivity was decreased and the BHR was2.12MPa which was limited in T4sample;②Pre-aging can suppress natural ageing andimprove BHR effectively, longer time can cause stronger effects. When pre-ageing timewas10min, the hardness change was only7.82HV, but the strength before baking wastoo high and the plasticity after baking was decreased;③Pre-straining alone cansuppress natural aging but had limited effect on BHR;④A combination of pre-strainingand pre-ageing can suppress natural ageing more effectively, higher pre-straining levelcombined with longer pre-ageing time can cause better suppression effect while couldnot enhance BHR effectively; Adding pre-straining before pre-ageing can retain thegood elongation while improving BHR obviously especially in10min pre-agedsample,the hardness change was only0.83HV and suppression of natural ageing andimprovement of BHR can be got without highly sacrifice its plasticity. The5%pre-straining and5min pre-ageing simple has best BHR of73.63MPa;⑤Suppression ofnatural ageing and improvement of bake hardening are more sensitive to pre-ageingthan pre-straining in the present alloy and pre-treatment has weak influence on plasticityin pre-treated samples. High temperature pre-ageing accelerated the precipitation ofGP(I) zone and′′phase and greatly reduced their activation energy so it clearly suppressed natural ageing and greatly improved BHR while pre-straining can not reducetheir activation energy. |
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